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Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209Bi82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209Bi82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.

The research reactor TRIGA Mainz is an ideal facility to provide neutron-rich nuclides with production rates sufficiently large for mass spectrometric and laser spectroscopic studies. Within the TRIGA-SPEC project, a Penning trap as well as a beamline for collinear laser spectroscopy are being installed. Several new developments will ensure high sensitivity of the trap setup enabling mass measurements even on a single ion. Besides neutron-rich fission products produced in the reactor, also heavy nuclides such as 235U or 252Cf can be investigated for the first time with an off-line ion source. The data provided by the mass measurements will be of interest for astrophysical calculations on the rapid neutron-capture process as well as for tests of mass models in the heavy-mass region. The laser spectroscopic measurements will yield model-independent information on nuclear ground-state properties such as nuclear moments and charge radii of neutron-rich nuclei of refractory elements far from stability. TRIGA-SPEC also serves as a test facility for mass and laser spectroscopic experiments at SHIPTRAP and the low-energy branch of the future GSI facility FAIR. This publication describes the experimental setup as well as its present status

We report the first confirmation of the predicted inversion between the 2p 3=2 and 1f 5=2 nuclear states in the g 9=2 midshell. This was achieved at the ISOLDE facility, by using a combination of insource laser spectroscopy and collinear laser spectroscopy on the ground states of 71;73;75 Cu, which measured the nuclear spin and magnetic moments. Much of the current effort in nuclear physics is focused on determining how the nuclear shell structure is changing in neutron-rich nuclei. This has been triggered by the observation of unexpected phenomena in several neutronrich isotopes, since radioactive ion beams of such nuclei became available more than three decades ago. In the lighter elements (e.g., He, Li, Be), neutron halos and skins were observed. Around the neutron-rich 32 Mg region an ''island of inversion'' was discovered. In the neutron-rich region towards doubly magic 78 Ni, a sudden drop in the position of the first excited 5=2 À state in 71;73 Cu isotopes was observed more than a decade ago [1]. The lowering of the 5=2 À energy from above 1 MeV in 69 Cu to 166 keV in 73 Cu suggested that this state might become the ground state in 75 Cu. The migration of this level, associated with the occupation of the 1f 5=2 single-particle orbital, was attributed to a strong attractive monopole interaction that becomes active when neutrons occupy the 1g 9=2 orbital [2]. Such monopole interactions exist also in near-stable nuclei, but their impact on the evolution of shell structure and shell gaps in far-from-stability nuclei remained unnoticed until recently [3]. Also in other neutron-rich regions dramatic monopole shifts were observed when valence neutrons and protons are occupying orbits having their orbital and spin angular momentum, respectively, aligned and antialigned. It is now understood that one of the physics mechanisms driving these monopole shifts is the tensor part of the residual nucleon-nucleon interaction [4]. A steep lowering of the 1=2 À level from about 1 MeV in 69 Cu down to 135 keV in 73 Cu has also been observed [5,6]. Thus this level is also a potential ground-state candidate in 75 Cu. While most shell-model interactions do reproduce a lowering of the 5=2 À level and predict an inversion with the normal 3=2 À ground state somewhere between 73 Cu and 79 Cu [4,[7][8][9][10], none of them reproduce the lowering of the 1=2 À state. Some significant physics mechanism is either omitted or seriously underestimated in each of the recently developed shell-model interactions. Therefore, experimental establishment of ground-and excited-state nuclear spins and the properties of their wave function (through spectroscopic factors, magnetic moments, transition moments, etc.) is a crucial step in PRL 103,

Abstract. A gas-filled segmented linear Paul trap has been installed at the focal plane of the high-resolution separator (HRS) at CERN-ISOLDE. As well as providing beams with a reduced transverse emittance, this device is also able to accumulate the ions and release the sample in bunches with a well-defined time structure. This has recently permitted collinear laser spectroscopy with stable and radioactive bunched beams to be demonstrated at ISOLDE. Surface-ionized 39,44,46 K and 85 Rb beams were accelerated to 30 keV, mass separated and injected into the trap for subsequent extraction and delivery to the laser setup. The ions were neutralized in a charge exchange cell and excited with a co-propagating laser. The small ion beam emittance allowed focussing in the ion-laser overlap region, which is essential to achieve the best experimental sensitivity. Fluorescent photons were detected by a photomultiplier tube as a frequency scan was taken. A gate (typically 7-12 μs wide) was set on the photomultiplier signal to accept the fluorescent photons within the time window defined by the bunch. Thus, using accumulation times of 100 ms, the dominant contribution to background due to continuous laser scattering could be reduced by a factor of up to 4 × 10 4 .

A recent measurement of the hyperfine splitting in the ground state of Li-like ^{208}Bi^{80+} has established a "hyperfine puzzle"-the experimental result exhibits a 7σ deviation from the theoretical prediction [J. Ullmann et al., Nat. Commun. 8, 15484 (2017)NCAOBW2041-172310.1038/ncomms15484; J. P. Karr, Nat. Phys. 13, 533 (2017)NPAHAX1745-247310.1038/nphys4159]. We provide evidence that the discrepancy is caused by an inaccurate value of the tabulated nuclear magnetic moment (μ_{I}) of ^{209}Bi. We perform relativistic density functional theory and relativistic coupled cluster calculations of the shielding constant that should be used to extract the value of μ_{I}(^{209}Bi) and combine it with nuclear magnetic resonance measurements of Bi(NO_{3})_{3} in nitric acid solutions and of the hexafluoridobismuthate(V) BiF_{6}^{-} ion in acetonitrile. The result clearly reveals that μ_{I}(^{209}Bi) is much smaller than the tabulated value used previously. Applying the new magnetic moment shifts the theoretical prediction into agreement with experiment and resolves the hyperfine puzzle.

We performed a laser spectroscopic determination of the 2s hyperfine splitting (HFS) of Li-like 209 Bi 80+ and repeated the measurement of the 1s HFS of H-like 209 Bi 82+ . Both ion species were subsequently stored in the Experimental Storage Ring at the GSI Helmholtzzentrum für Schwerionenforschung Darmstadt and cooled with an electron cooler at a velocity of ≈ 0.71 c. Pulsed laser excitation of the M 1 hyperfine-transition was performed in anticollinear and collinear geometry for Bi 82+ and Bi 80+ , respectively, and observed by fluorescence detection. We obtain ∆E (1s) = 5086.3(11) meV for Bi 82+ , different from the literature value, and ∆E (2s) = 797.50(18) meV for Bi 80+ . These values provide experimental evidence that a specific difference between the two splitting energies can be used to test QED calculations in the strongest static magnetic fields available in the laboratory independent of nuclear structure effects. The experimental result is in excellent agreement with the theoretical prediction and confirms the sum of the Dirac term and the relativistic interelectronic-interaction correction at a level of 0.5% confirming the importance of accounting for the Breit interaction.Quantum electrodynamics (QED) is generally considered to be the best-tested theory in physics. In recent years a number of extremely precise experimental tests have been achieved on free particles as well as on bound states in light atomic systems. For free particles, the g-factor of the electron measured with ppb-accuracy [1] constitutes the most precise test, sensitive to the highest order in α [2]. In atomic systems the QED deals with the particles bound by the Coulomb field, what makes high-precision QED calculations more complicated. The bound-state QED (BS-QED) effects in light atomic systems are expanded in parameters Zα and m e /M in addition to α, where Z is the atomic number and m e and M are the electron and nuclear masses, respectively. The parameter Zα characterizes the binding strength in the Coulomb field of the nucleus, while the mass ratio m e /M is introduced for the nuclear recoil effects. Hence, tests of BS-QED are complementary to QED tests of the properties of free particles. The investigation of H-like systems with increasing charge provides the opportunity to systematically increase the influence of the binding effect.One of the most accurate test of BS-QED on low-Z ions is the measurement of the g-factor of a single electron bound to a Si nucleus [3]. Entering the regime of highly charged heavy ions like Pb 81+ , Bi 82+ or U 91+ the electron binding energy becomes comparable to the rest-mass energy and the parameter Zα can not be employed as an expansion parameter anymore. In other words, the extremely strong electric and magnetic fields in the close surrounding of the heavy nucleus require the inclusion of the binding corrections in all orders of Zα. Hence, BS-QED in this regime requires a very different approach and new tools to calculate the corresponding corrections, usually referred to as strong-fi...

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